1. Field of the Invention
The present invention relates to organic light emitting diode (OLED) display device and a method of fabricating the same and, more particularly, to an OLED display device in which OLED pixels are patterned through a photolithography process, and a method of fabricating the same.
2. Description of the Related Art
Recently, as interest in information displays has been on the rise and demand for the use of portable information media has been increased, lightweight flat panel displays (FPDs) substituting cathode ray tubes (CRTs) as existing display devices have been actively researched and commercialized.
In the FPD fields, a liquid crystal display (LCD) device, which is lighter and consumes less power, has been spotlighted; however, since an LCD device is a light receiving device, rather than a light emitting device, having shortcomings of brightness, contrast ratio, and a viewing angle, and the like, so a development of a new display device that may overcome such drawbacks has been actively made.
An LED display device, one of new display devices, is a self-luminous type device, which thus is excellent in a viewing angle and contrast ratio, is lighter and thinner because it does not need a backlight, and is advantageous in terms of power consumption, relative to an LCD device. In addition, an OLED display device can be driven by a DC and at a low voltage, has a fast response speed, and is especially advantageous in terms of fabrication costs.
Unlike an LCD device or a plasma display panel (PDP), deposition and encapsulation are the whole of a fabrication process of an OLED display device, so the fabrication process is very simple. Also, when the OLED display device is driven according to an active matrix scheme in which each pixel has a thin film transistor (TFT) as a switching element, the same luminance can be obtained although a low current is applied, so, advantageously, the OLED display device consumes low power, has a high pitch (or high definition or high resolution), and can be increased in size.
Hereinafter, a basic structure and operational characteristics of an OLED display device will be described in detail with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating a light emission principle of a related art OLED display device.
As shown in FIG. 1, a related art OLED display device includes an OLED. The OLED includes organic compound layers 301, 30b, 30c, 30d, and 30e formed between an anode 18 as a pixel electrode and a cathode 28 as a common electrode.
Here, the organic compound layers 30a, 30b, 30c, 30d, and 30e include a hole injection layer 30a, a hole transport layer 30b, an emission layer 30c, an electron transport layer 30d, and an electron injection layer 30e. 
When a driving voltage is applied to the anode 18 and the cathode 28, holes which have passed through the hole transport layer 30b and electrons which have passed through the electron transport layer 30e move to the light emission layer 30c to form excitons, and as a result, the light emission layer 30c emits visible light.
In the OLED display device, the pixels each having the OLED having the foregoing structure are arranged in a matrix form and selectively controlled by a data voltage and a scan voltage to display an image.
The OLED display device is divided into a passive matrix type OLED display device and an active matrix type display device using TFTs as switching elements. Among them, in the active matrix type OLED display device, TFTs as active elements are selectively turned on to select pixels and emitting of pixels is maintained by a voltage maintained in a storage capacitor.
FIG. 2 is an equivalent circuit diagram of a pixel in a related art OLED display device. Namely, FIG. 2 illustrates an example of an equivalent circuit diagram of a pixel having a related art 2T1C (including two transistors and one capacitor) in an active matrix type OLED display device.
Referring to FIG. 2, a pixel of an active matrix type OLED display device includes an OLED, a data line DL and a gate line GL crossing each other, a switching TFT SW, a driving TFT DR, and a storage capacitor Cst.
Here, the switching TFT SW is turned on in response to a scan pulse from the gate line GL to conduct a current path between a source electrode and a drain electrode thereof. During an ON-time period of the switching TFT SW, a data voltage from the data line DL is applied to a gate electrode of the driving TFT DR and the storage capacitor Cst by way of the source electrode and drain electrode of the switching TFT SW.
Here, the driving TFT DR controls a current flowing in the OLED according to the data voltage applied to the gate electrode thereof. The storage capacitor Cst stores a voltage between the data voltage and a low potential power source voltage VSS and uniformly maintains it during one frame period.
Recently, interest in a middle and large display market, beyond a small display panel for portable devices, has increased and a white organic light emitting diode (W-OLED) has come to prominence as a technique satisfying such market demand. A W-OLED uses color filters to implement red, green, and blue colors. Also, in order to achieve a large OLED display device, the development of a transistor which stably operates and has durability by securing constant current characteristics, as a driving thin film transistor (TFT) has been requested.
Thus, an oxide TFT in which an active layer is formed of an oxide semiconductor has been developed. Hereinafter, a structure of a related art W-OLED display device employing an oxide TFT will be described in detail.
FIG. 3 is a cross-sectional view schematically illustrating a structure of a related art W-OLED display device. The related art W-OLED display device illustrated in FIG. 3 has a color filter on TFT (COT) structure in which a color filter is formed on a lower array substrate.
Referring to FIG. 3, the W-OLED display device having the COT structure implement red, green, and blue colors by using a color filter 17 formed on an array substrate 10. In detail, the array substrate includes a plurality of gate lines (not shown) and data lines (not shown) defining a plurality of pixel regions, TFTs formed at crossings of the gate lines and the data lines, the color filter 17 and a pixel electrode 18 formed in each of the pixel regions.
The TFT includes a gate electrode 21 connected to the gate line, a source electrode 22 connected to the data line, and a drain electrode 23 connected to the pixel electrode 18. Also, the TFT includes a gate insulating layer 15a for insulating the gate electrode 21 and the source and drain electrodes 22 and 23, and an active layer 24 made of an oxide semiconductor and forming a conductive channel between the source electrode 22 and the drain electrode 23 by a gate voltage supplied to the gate electrode 21.
An oxide semiconductor used in the oxide TFT has a weak bonding structure. Thus, in order to prevent damage to a back channel region, an etch stopper 25 is required to be additionally formed on the active layer 24, having shortcomings in that a corresponding process is additionally performed. In this case, the etch stopper 25 is applied to secure stability of the back channel in a bottom gate structure.
A protective film 15b is formed at an upper portion of the TFT configured as described above, and the red, green, and blue color filter 17 is formed on the protective film 15b of the pixel region. An overcoat layer 15c is formed on a front surface of the array substrate 10 on which the color filter 17 is formed, in order to compensate for a step between the color filter 17 and the TFT.
The pixel electrode 18 is formed on the overcoat layer 15c. In this case, the pixel electrode 18 is electrically connected to the drain electrode 23 through a contact hole.
Here, although not shown, a partition is formed on the array substrate 10 on which the pixel electrode 18 is formed, and a white organic light emitting layer is formed on the array substrate 10 on which the partition is formed. A common electrode as a cathode is formed on the organic light emitting layer.
In the related art W-OLED display device having the etch stopper configured as described above, in order to form up to the pixel electrode, at least 11 masks such as a gate wiring (i.e., the gate electrode and the gate line), the active layer, the etch stopper, a gate contact, a data wiring (i.e., the source electrode, the drain electrode, and the data line), the protective layer, the red, green, and blue color filter, the overcoat layer, the pixel electrode, and the like, are required, and parasitic capacitance is large due to interlayer superposition.
Also, as mentioned above, the etch stopper is applied in order to secure stability of the back channel, but it is difficult to secure reliability characteristics due to light introduced from upper and lower portions of the active layer, and although a high temperature thermal treatment is required to improve reliability, it is difficult to apply copper (Cu) as a gate wiring.
For reference, in the related art OLED display device, two or more TFTs including a driving TFT and a switching TFT are present in a single pixel region, and the foregoing gate contact is required in order to connect the gate electrode of the driving TFT and the drain electrode of the switching TFT.